1. Field of the Invention
The present invention relates to an objective-lens driving device in which a lens holder on which an objective lens is mounted is displaceably and elastically supported by an actuator frame.
2. Description of the Related Art
As is well known, in an objective-lens driving device in an optical head for optically reading a signal from or writing a signal into a signal storage medium such as a disc, a lens holder on which an objective lens is mounted is displaceably supported by an actuator frame, and focusing coils, tracking coils, and, as necessary, tilt coils are mounted on the lens holder. By arranging the respective effective regions of the foregoing driving coils in a predetermined magnetic field created by a magnetic circuit, the objective lens is driven in response to respective signals supplied to the driving coils.
The method of mounting driving coils on a lens holder includes affixing driving coils that have preliminarily been wound to the lens holder, winding the driving coils directly around the lens holder as a bobbin, and affixing printed coils to the lens holder after configuring the driving coils with the printed coils formed as strip conductors on printed boards. The method of winding the driving coils directly around the lens holder as a bobbin has an advantage in terms of driving performance and assembly efficiency; in many cases, employing this method for an objective-lens driving device is desirable.
Meanwhile, as an objective-lens driving device, a configuration is known in which tracking coils rectangularly wound in such a way that the center axes of the windings are perpendicular to the tracking direction or the focusing direction are mounted on the lens holder in such-a way as to oppose the magnetic-pole surfaces of magnets configuring a magnetic circuit.
In the objective-lens driving device, regions parallel to the focusing direction are formed in two sides of each tracking coil; therefore, by making the polarities, at the two sides, of magnetic flux generated by the magnets contrary to each other, the two sides can serve as regions for effectively driving the objective lens in the tracking direction, thereby enhancing driving sensitivity in the tracking direction.
Meanwhile, in objective-lens driving devices, in many cases, by arranging an objective lens in the center of the lens holder, arranging a pair of magnets that form a magnetic circuit so as to flank the lens holder, in a direction perpendicular to the tracking direction or the focusing direction, and mounting on the lens holder a pair each of driving coils opposing the magnet, the center of driving force that acts on the lens holder is made to coincide with that of the objective lens, whereby satisfactory performance can be obtained.
In recent objective-lens driving devices, in order to make the optical axes of laser beams radiated onto a disc perpendicular to the disc so as to cope with an increase in disc density, tilt coils are mounted on the lens holder so that the objective lens can be driven obliquely in a direction in which the disc slants; i.e., in the radial skew direction.
Meanwhile, in the case of an objective-lens driving device in which tracking coils wound in such a way that the center axes of the windings are perpendicular to the tracking direction or the focusing direction are mounted on the lens holder, in general, the tracking coil and a focusing coil are arranged in a superimposed fashion so that the same magnetic field generated by a magnetic circuit can be utilized; therefore, an objective-lens driving device such as this cannot employ a configuration in which the tracking coils are wound directly around the lens holder, whereby the objective-lens driving device has had a disadvantage in assembly efficiency; and because, due to superimposition of the tracking coil and the focusing coil, the magnetic gap cannot be narrowed, the objective-lens driving device has had a disadvantage in enhancement of driving sensitivity.
Additionally, in the case of an objective-lens driving device in which the objective lens can be driven obliquely in the radial skew direction, in general, two pairs of focusing coils and tilt coils or two focus coils are arranged so as to flank the objective lens in the tracking direction and the magnetic circuit is configured in such a way that respective coils at both sides of the objective lens are arranged in magnetic fields whose polarities are contrary to each other. An objective-lens driving device such as this cannot employ a configuration in which the two pair of focusing coils and tilt coils or the two focusing coils cannot be wound directly around the lens holder; additionally, because, in order to use the tracking coils effectively, the tracking coils are often inserted between the focusing coils, there cannot be employed a configuration in which the tracking coils are wound directly around the lens holder, whereby there has been a problem in terms of assembly efficiency. Moreover, with regard to the focusing coils at both sides of the objective lens, the focusing coil and the tracking coil are often arranged in a superimposed fashion; therefore, the objective-lens driving device has had a disadvantage in terms of driving sensitivity.